The invention relates to the field of microfabrication of glass ceramic materials. More particularly, the present invention relates to the manufacture of structure within photoactivable glasses and ceramics using variable laser exposures.
Glass is a highly versatile and functional-material that can be designed with specific properties, such as high compressive strength and durability, corrosion resistance and chemical inertness, low thermal and electrical conductivity, negligible porosity and biocompatibility. Consequently, glass has received significant attention for potential applications in a variety of scientific and industrial fields, including aerospace engineering, photonics, optoelectronics, biology and biochemistry, and microelectromechanical systems (MEMS) design. These applications require integrated and functional structures that range in scale from the microscale of less than 100 xcexcm, to the mesoscale of 100 xcexcm to 10 mm, and to the macroscale of greater than 10 mm. For example, integrated photonic and biofluidic devices have feature dimensions varying from submicrons to the hundreds of microns. In the intermediate mesoscale domain, arrays of glass microfluidic channels and precision volume containers, such as microtiter plates, are often utilized to facilitate the automation of chemical and biochemical analysis.
The ability to fabricate intricate microstructures in glass materials is essential to meet the increasing demands of future microtechnology development. These advanced design requirements will involve the fabrication of high and low aspect ratio structures with variegated heights on a common substrate. Microscale and mesoscale structure fabrication in glass materials have typically been achieved using optical lithographic patterning and chemical etching, mechanical micromilling and direct thermal ablation with ultrafast lasers. Although lithography and mechanical cutting methods permit the formation of two dimensional patterns, these approaches do not allow the fabrication of true three dimensional structures with high and low aspect ratios.
In principle, conventional material removal methods can be applied to the cofabrication of two-dimensional microstructures with different aspect ratios on a shared substrate. However, the processing steps are numerous and costly for these approaches to be practical and economical. For microelectromechanical systems applications requiring concurrent and proximal high aspect ratio and low aspect ratio features, the fabrication solution generally involves the use of an elaborate masking sequence. The sequential masking steps are intended to protect physically the low aspect ratio structures during the long duration etching time needed to form the high aspect ratio structures. An alternative approach concerns the introduction of dopants and impurities into the substrate that selectively alter the local etching rate and permit the concurrent formation of high and low aspect ratio features. Another alternative is to generate the high and low aspect ratio structures on separate substrates. Following preparation, the substrates can be joined or packaged together to merge the variegated aspect ratio structures. Unfortunately, these alternative solutions are time consuming, difficult and expensive to implement. Laser ablation or micromilling methods can fabricate continuously variable aspect ratio structures in glass or ceramic materials. However, the fabricated structures suffer from residual thermal induced effects, such as structural stress, cracks and optical defects.
A prior method demonstrated in the related application teaches fabricating embedded structures in glass and ceramic materials that are photostructurable glass ceramic material, commonly called pyrocerams or photocerams. One photostructurable material has been commercially available under the tradename Foturan(trademark). This photostructurable material is a preferred photoceramic material, but other glass ceramic materials are also suitable for photostructuring of embedded structures using laser exposure. A predetermined laser energy dose and wavelength are applied to the Foturan photoceramic material. The laser is a pulsed ultraviolet laser. The laser provides a pulsed laser beam using a lens defining a beam waist at a focal depth that is moved during exposure relative to the exposed material. The choice of the UV wavelength is critical. The wavelength is preferably at the very edge of the spectral region where photoceram transitions from being strongly absorbing to weakly absorbing. In the weakly absorbing spectral region, the wavelength of the laser is outside the strong absorption band of the photoceramic material. Hence, the absorption of radiation is very small so that the process is photon inefficient but enables the controlled focused exposure of any volume including an embedded focal volume defining an embedded three-dimensional structure. The focused beam illuminates the material with the intensity peaking in the focal volume. The number of pulses and pulse fluence controls the amount of the exposure dose so that the exposure outside the focal depth, that is, outside the depth of the optical field in a collateral volume, is insufficient for conversion of the material to the soluble crystalline phase. Within the depth of focus region, that is, the focal volume, the combined effect of the focused laser beam fluence in Joules/cm2, and the dose in terms of the number of laser pulses is beyond a critical dose that is required for conversion to the crystalline phase. The focused pulsed ultraviolet laser and a computer-controlled sample positioning stage and shutter provides motion controls for moving the material relative to the focus laser beam during selective exposure of the focal volume. True three-dimensional patterns can be formed by moving the sample using an XYZ positioning stage in XYZ directions. Motion and shutter operations are both computer controlled. For example, laterally moving the workpiece in the XY plane can create an embedded tunnel within the material, while moving vertically for adding via openings to the end sections of a tunnel so as to undercut the structure above leaving an anchored but suspended structure. The result is an embedded microstructure or an exposed pixel defined in the focal volume.
The method can be used to create one or more stacked embedded structures. There is no critical exposure above and below depth of focus. The material is only critically exposed in the focal volume region where the administered laser dose is above a critical dose. Repeated exposures at different depth of focus enable the formation of stacked embedded structures. Precise structural definition is realized for creating one or more embedded structures because collateral volume regions above and below the focal volume at the depth of focus do not accumulate the critical exposure dose. The critical dose is based on sufficient per pulse fluence that is the energy per unit area in a single pulse and the number of pulses. For a given laser pulse width and wavelength, the per pulse fluence is proportional to irradiance. The exposure process is a nonlinear optical process, that is, the critical dose is a nonlinear function of the per pulse laser fluence. That is, the critical dose required for conversion to the crystalline phase is both a function of the per pulse fluence and the number of applied pulses. The dose dependence is nonlinear in per pulse fluence and is cumulative. The most intense portion of the focused pulsed laser light is sufficient to deliver the critical dose over a predetermined number of pulses. Exploiting the nonlinear aspect of the exposure process allows for creation of stacked structures at any desired focal depth. The wavelength is in the weak absorption region so that a critical dose is not delivered to the collateral volume where the pulsed laser light is not focused and not as intense as in laser focal volume where the laser light provides a critical dose. Hence, the pulsed laser light has a wavelength in the weak absorption region of the photoceram so that the pulsed laser light passes through the collateral volume without delivering a critical dose and without producing crystallization outside the focal depth. The pulsed laser light is focused and intensely converges for accumulation of the critical dose only in the focal volume for selective critical dose exposure at the focal depth in the photoceram enabling precise embedded volumetric critical dose exposure. Exposing the photoceram material with higher fluence focused light provides a sharp selective contrast between the insufficiently exposed collateral volume and the sufficiently exposed focal volume. During fabrication, the method provides a trade off between laser fluence and the number of applied pulses. The critical dose of pulsed UV light provides an embedded growth of an etchable crystalline phase of the photoceram. By exposing bulk photoceram material to a fluence gradient for a variety of pulse train lengths, and by measuring the dimensions of the etched region, the proper critical dose can be determined in terms of wavelength, intensity and the number of pulses.
A crystallization boundary is created in the photostructurable glass where the critical dose is and is not exceeded. Above the critical dose, the photostructurable glass is crystallized for forming a latent image in the focal volume. Below the critical dose, no image is formed, and hence, no crystallization occurs in that region of the photostructurable glass. The crystallization boundary separates the collateral volume from the focal volume. The crystallized glass that was subjected to a dose higher than critical dose in the focal volume is etched away from the collateral volume of photostructurable material. The under-exposed, uncrystallized photostructurable glass in the collateral volume that did not have a critical exposure remains after etching. The critical dose is required to create a density of nuclei large enough to result in an interconnected network of crystallites. The crystallites are etched away in a subsequent etching process. The density of nucleation sites is proportional to the dose and the critical dose will be a function of the material composition and process parameters. For a given number of pulses, the critical dose will correspond to a critical fluence. When a focal volume of embedded photoceram material is exposed above the critical dose, the focal volume can then be developed, etched, and vacated, the vacant cavity defining the embedded structure.
This prior method enables the formation of embedded microstructures, microcavities in a particular class of glass and ceramic materials. The method also enables the patterned undercutting of unexposed structures resulting in the fabrication of suspended or supported glass and ceramic microstructures. A micromachining station includes a pulsed laser that provides a focused laser beam and the workpiece including the photoceram material. The laser can be moved relative to a stationary workpiece, or equivalently, the workpiece can be moved relative to the laser beam, both according to a predefined computer program. The exposure process is maskless and amenable to rapid batch fabrication of the embedded structures because the only serial aspect of the process is the exposure step. A parallel batch fabrication process can be used in the actual material removal step. Depth control of the laser light is achieved by the proper choice of exposure wavelength and focusing optics. As the laser wavelength is tuned into the weak absorption end of the UV absorption band of the photoceram material, the absorption of laser light decreases in the collateral volume and the penetration depth increases into the focal volume for crystallization of embedded structures. The pulsed laser ultraviolet light beam can be shaped resulting in structures that will retain the beam shape. For example, a collimated beam can result in a cylindrical hole. A focused beam can result in either a cone section, or a hyperboloid structure.
The prior method is a direct-write three dimensional (3D) volumetric lithography method for selective removal of the embedded photoceram material. The method is photon efficient in the sense that the laser must only provide the energy to form a latent image. The method does not need to provide the energy to break atomic bonds and remove unwanted material from the workpiece. Direct write tools, such as computer-aided manufacturing programs and fixtures coupled to a pulsed ultraviolet laser are used in a micromachining station for rapid batch processing and enhanced depth control. The pattern exposure and dissolving steps can be done across an entire wafer for batch processing. The prior method achieved the intended purpose, of defining a critical dose, for the creation of an embedded structure. However, the method relies upon flood illumination at a maximum intensity for rapid critical dose exposure for forming exposed volumes having a single etch rate with the exposed volumes then being etched using traditional multiple photomasking steps for creating respective differing aspect ratio features. The prior method does not enable cost effective etching and variable aspect ratio processing of photosensitive materials. These and other disadvantages are solved or reduced using the new invention.
An object of the invention is to provide a method for forming high and low aspect ratio features in a photosensitive material.
Another object of the invention is to provide a method for forming high and low aspect ratio features in a photosensitive material through variable laser intensity exposure without the need for employing protective masks.
Yet another object of the invention is to provide a method for forming high and low aspect ratio features in a photosensitive material through variable laser intensity exposure resulting in differing etch rates of respective volumes of the photosensitive material without the need for employing protective etch masks.
Still another object of the invention is to provide a method for forming high and low aspect ratio features in a photosensitive material during a single etch process step through variable laser intensity exposure of the photosensitive material resulting in differing etch rates of respective volumes of the photosensitive material.
The invention is directed to a material processing technique that permits the sequential volumetric patterning of variegated and high and low aspect ratio structures on a common glass substrate. A complete ensemble of patterned structures can be simultaneously released using a single, timed-interval etching technique. The variable laser exposure method offers direct-write serial patterning with batch chemical processing, and permits the fabrication of complex structures and features on a single wafer. Precise variation of the laser irradiance is used during material patterning. The controlled variation of the laser irradiance is used to selectively alter the chemical etch rate of the processed regions in the glass so that high and low aspect ratio features can be formed using a single chemical etch step. The method is preferably applied to photoceramic or photosensitive glass for fabricating continuously variable aspect ratio structures on a common substrate. The required relief structures can be realized without using any masking layers for cost effective batch fabrication. Adjacent microstructures with aspect ratios of 2:1 to 30:1 can be laser patterned through variable exposure processing and concurrently fabricated on a shared single glass wafer following a single chemical etch step. The variable exposure method enables the conversion of computer assisted design patterns and corresponding laser irradiance information into realized microfabricated structures in glass or ceramic form that retain precise feature sizes within desired, predetermined dimensions. The variable laser method offers precision glass or ceramic material pattering of mesoscale devices that contain microscale features with variable aspect ratios. For example, the method can be used to fabricate various millimeter-sized components that may be useful in far infrared or terahertz devices that require high aspect ratio mesoscale structures with microscale features. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.